In the field of non-impact printing, the most common types of printers have been the thermal printer and the ink jet printer. When the performance of a non-impact printer is compared with that of an impact printer, one of the problems of the non-impact machine has been the control of the printing operation. As is well-known, the impact operation depends upon the movement of impact members such as wires or the like, and which are typically moved by means of an electromechanical system which may, in certain applications, enable a more precise control of the impact members.
The advent of non-impact printing, as in the case of thermal printing, brought out the fact that the heating cycle must be controlled in a manner to obtain maximum repeated operations. Likewise, the control of ink jet printing must deal with rapid starting and stopping movement of the ink fluid from a supply of the fluid. In each case of non-impact printing, the precise control of the thermal elements and of the ink droplets is necessary to provide for both correct and high-speed printing.
In the matter of ink jet printing, it is extremely important that the control of the ink droplets be precise and accurate from the time of formation of the droplets to depositing of such droplets on paper or like record media and to make certain that a clean printed character results from the ink droplets. While the method of printing with ink droplets may be performed either in a continuous manner or in a demand pulse manner, the latter type method and operation is disclosed and is preferred in the present application as applying the features of the present invention. The drive means for the ink droplets is generally in the form of a crystal or piezoelectric type element to provide the high-speed operation for ejecting the ink through the nozzle while allowing time between droplets for proper operation. The ink nozzle construction must be of a nature to permit fast and clean ejection of ink droplets from the print head.
Additionally, in an ink jet printer, it is considered a basic requirement to provide some type of low ink indication to signal the impending exhaustion of the ink supply. Contrasted with an impact printer wherein an examination of the printed output provides an indication of low ink by reason of the printed output becoming faint or difficult to read, the ink jet printer will cease operation when the ink supply is depleted.
The manner of sensing low ink can be performed electrically or optically and the low ink condition can be signalled to the operator. However, this manner of sensing is more expensive and may not be feasible in certain types of ink jet printer configurations.
Representative documentation in the field of ink jet printing and ink supply indication means includes U.S. Pat. No. 4,148,041, issued to G. Rosenstock on Apr. 3, 1979, which discloses the use of a flushing liquid or wetting agent which does not mix with the printing liquid in a manner and for the purpose of eliminating air bubbles from the printing operation. A preferred form of the flushing liquid is an isoparaffin.
U.S. Pat. No. 4,183,031, issued to E. L. Kyser et al. on Jan. 8, 1980, discloses an ink supply connection for automatically closing off the ink supply line and includes an ink supply pressure sensor for sensing changes in pressure in the system and a valve operated by the sensor to supply ink to the system upon reduction of the pressure below a predetermined level.
U.S. Pat. No. 4,196,625, issued to H. Kern on Apr. 8, 1980, discloses a device for monitoring the ink supply in ink recording devices wherein electrodes are wetted by the ink and a circuit detects the electrical resistance between the electrodes and triggers a display device when the amount of fluid falls below a specific volume.
U.S. Pat. No. 4,202,267, issued to J. Heinzl et al. on May 13, 1980, discloses a device for monitoring the ink supply in an ink printer wherein electrodes are wetted by the ink and a circuit monitors the electrical resistance which varies with the amount of ink in the reservoir. Additional circuit means may be connected to the monitoring means for actuating a warning device prior to triggering a display element indicating the exhaustion of the ink supply.